1. Field of the Invention
The present invention relates to improved methods and compositions for sealing oil containing subterranean zones penetrated by well bores.
2. Description of the Prior Art
In the drilling of oil and gas wells using the rotary drilling method, drilling fluid is circulated through the drill string and drill bit and then back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean zones through which the well bore is drilled and circulates cuttings out of the well bore. During such drilling, subterranean vugs, fractures and other thief zones are often encountered whereby the drilling fluid circulation is lost and drilling operations must be terminated while remedial steps are taken. Also, when a subterranean zone is penetrated containing fluids under pressure which exceeds the hydrostatic pressure exerted on the zone by the drilling fluid, formation fluid cross-flows and/or underground blowouts can and often do occur.
Heretofore, a variety of compositions have been developed and used for combating lost circulation, cross-flow and underground blow-out problems. However, such compositions have often been unsuccessful due to delayed and inadequate viscosity development by the compositions. For example, a variety of compositions containing hydraulic cement or the like have been used in attempts to stop lost circulation. The lost circulation is usually the result of encountering weak subterranean zones that contain natural fractures or are fractured by drilling fluid pressures and rapidly break down. When a cement or other slow setting composition is squeezed into the zone, the delay in developing high viscosity allows the cement composition to be diluted and displaced into the zone whereby it by-passes the fractures and vugs causing the lost circulation. The same type of problem often occurs when cross-linked hydrated gels and other similar plugging compositions are utilized.
More recently, water-based compositions for sealing subterranean zones and methods have been developed for sealing subterranean zones containing water-based or oil-based drilling fluids. The compositions for sealing subterranean zones containing oil-based drilling fluids are basically comprised of water, an aqueous rubber latex, an organophilic clay, and other components. When such a sealing composition is pumped into a subterranean zone and contacts oil-based fluids therein, the composition viscosities to a thick mass very quickly and plugs fractures, vugs and other thief zones in the subterranean zone. A problem sometimes encountered in the use of the above described sealing composition is that the mixture can solidify prematurely prior to contacting oil-based fluids in the subterranean zone to be sealed. Additionally, the aqueous rubber latex and the latex stabilizing surfactants utilized in the sealing composition are costly and make the use of the sealing composition expensive.
Thus, there is a need for improved methods and compositions for sealing subterranean zones that develop high viscosity in a few seconds or minutes and are relatively inexpensive to use.